High molecular weight copolyesters containing both neopentyl glycol (NPG) and 2,2,4,4-tetramethyl 1,3-cyclobutanediol (TMCD) are difficult to prepare using conventional catalysts (e.g. titanium and/or tin) and polymerization conditions. TMCD can increase the glass transition temperature (Tg) in copolyesters made from dimethyl terephthalate (DMT).
Producing an amorphous copolyester having a high Tg is desirable for certain end use applications where the polymer must remain dimensionally stable under heat. While TMCD can raise the Tg of an amorphous copolyester, it becomes difficult to insert TMCD residues into the backbone of the copolyester in the melt phase polycondensation to produce a polymer with high Tg. Using an excess of TMCD to incorporate the desired amount of TMCD into the backbone of the copolyester results in increased costs for use of the excess TMCD.
However, a polyester made using only TMCD as the glycol has such a high melt viscosity that it is difficult to make and process. Therefore, other glycol comonomers are used to lower the melt viscosity of the polymer melt and to also improve other properties such as toughness. One glycol that has been used is 1,4-cyclohexanedimethanol (CHDM). Copolyesters made from DMT, CHDM and TMCD have had commercial success, but can suffer from decreased thermal stability due to degradation reactions with the CHDM component, meaning that the polymer loses molecular weight during high temperature processing of the polymer in melt extrusion. Furthermore, high processing temperatures during the manufacture of the polymer, especially in the final polymerization reactor, also called the finisher, are desirable to produce polymers from the melt that have a high molecular weight, typically measured as the polymer's intrinsic viscosity (It.V). However, a polymer with poor thermal stability at the finisher reaction temperatures makes it difficult to adequately build the IV (0.55 dL/g or more) needed for many end use applications.
Thus, there remains a need to develop a copolyester containing TMCD that has a good balance of high Tg (at least 90° C.), good IV (0.55 dL/g or more), has an acceptable level of TMCD incorporation into the backbone of the copolyester polymer, and has good thermal stability.